1. Field of the Invention
The present invention relates to a method for fabricating a tungsten plug in a contact hole of a highly integrated semiconductor device, and more particularly to a method for fabricating a tungsten plug by using several tungsten film deposition steps capable of obtaining a tungsten plug with a high density.
2. Description of the Prior Art
In fabrication of a multilayer wiring of a highly integrated memory device or a logic device, an upper metal wiring is in contact with a lower conductive layer through a contact hole formed in an insulating film to have a vertical shape so that it is connected with the lower conductive layer. An increased integrity of such device results in an increase in aspect ratio (the ratio of height to width) of the contact hole. As a result, the step coverage of an upper layer deposited may be deteriorated. The contact hole may be incompletely filled with a metal layer used as the upper layer. These phenomenons cause a problem of an increase in contact resistance.
For solving such problem, a proposal for the use of a contact plug made of a metal layer filling the contact hole has been made.
For fabricating such a contact plug, an insulating layer is formed over a lower conductive layer comprised of, for example, a silicon layer or a metal layer. A predetermined portion of the insulating layer is then etched, thereby forming a contact hole. Over the entire exposed surface of the resulting structure, a blanket tungsten layer is thickly deposited. The tungsten layer is then etched back at its portion disposed over the remaining insulating film so that it remains only in the contact hole. Thus, a contact plug is obtained.
However, the blanket tungsten layer thickly deposited as mentioned above may have a low density resulting in key holes in the tungsten plug. Furthermore, the tungsten layer may have a rough surface, so that subsequent steps may be unstably performed.